Turn off the Instant Previews feature

By default, the Instant Previews feature is turned on.

On the home screen, swipe down from the top of the screen.

Tap Settings > Notifications.

Set the Instant Previews switch to Off.

Note: You can display only an application's priority messages as Instant Previews. On the
Notifications Settings screen, tap the application and in the Instant
Previews drop-down list tap Priority
Only.